Relationship between ultrastructural profiles of membrane systems involved in excitation-contraction coupling and signal transduction in skeletal muscle cells following exercise training

t小管和SR在结构和功能上以Ca^<2+>释放单元（cru）连接，称为三联体。我们研究了短期去神经支配后再神经支配对快速抽动（FT）和慢抽动（ST）肌纤维cru超微结构的影响。坐骨神经冻结导致3天内神经肌肉传递完全缺失，随后缓慢恢复，但直接刺激引起的抽搐力没有下降。后一种测量表明在这段时间内没有明显的萎缩。用高压电子显微镜观察了骨骼肌纤维的膜系统。[^3H]尼群地平结合检测二氢吡啶受体（DHPR）表达水平。Ca^<2+>泵送自由肌浆网域不受去神经支配的影响，但Ca^<2+>释放域显著增加，尤其是在FT纤维中。这种增加的证据是SR和t小管之间的接触面积增加了一倍，因此可以看到由ST和t小管的多个相互作用层形成的五元和七元连接取代了正常的三元排列。五音和七音的频率增加和减少与去神经支配和再神经支配平行，但有延迟。免疫荧光和电镜观察显示，DHPR和ryanodine受体簇存在于五联体和七联体交界处。在FT和ST纤维中观察到[^3H]尼群地平结合成分的水平与五联体和七联体的频率之间存在显著的正相关，这表明dhpr的过度表达伴随着额外连接接触的建立。有可能是由于肌肉失用而出现了多位点的五聚体和七聚体形成，以补偿正常的cru，从而增加了SR肌肉收缩释放的Ca^<2+>。

The t-tubules and SR are structurally and functionally joined at Ca^<2+> release units(CRUs), known as triads. We have studied the effects of short term denervation followed by reinnervation on the ultrastructural architectures in CRUs of fast-twitch(FT) and slow-twitch(ST) muscle fibers. Ischiadic nerve freezing resulted in total lack of neuromuscular transmission for 3 days followed by a slow recovery, but no decline in twitch force elicited by direct stimulation. The latter measurements indicate no significant atrophy within this time frame. The membrane systems of skeletal muscle fibres were visualized using a high voltage electron microscope. [^3H]nitrendipine binding was used to detect levels of dihydropyridine receptor(DHPR) expression. The Ca^<2+> pumping free sarcoplasmic reticulum domains were not affected by the denervation, but the Ca^<2+> release domains were dramatically increased, particularly in the FT fibres. The increase is evidenced by a doubling up of the areas of contacts between SR and t-tubules, so that in place of the normal triadic arrangement, pentadic and heptadic junctions, formed by multiple interacting layers of ST and t-tubules are seen. Frequency of pentads and heptads increases and declines in parallel to the denervation and reinnervation but with a delay. Immunofluorecence and electron microscopy observations show presence of DHPR and ryanodine receptor clusters at pentads and heptads junctions. A significant positive correlation between the level of [^3H]nitrendipine binding component and the frequency pentads and heptads was observed in both the FT and ST fibres indicating that overexpression of DHPRs accompanies the build up extra junctional contacts. It is possible that the multiple sites of pentad and heptad formation appear in response to muscle disuse to compensate for the normal CRUs, thus acting to increase the released Ca^<2+> for muscular contraction from the SR.